Implement coro::Future<void>::toFuture()

Summary: When converting to folly::Future we have to perform the void -> folly::Unit conversion.

Differential Revision: D8463177

fbshipit-source-id: 96312d0447dfc381a0ee586030f21b9cdbf4bee4

folly/experimental/coro/Future.h

@@ -78,10 +78,7 @@ class Future {
     return get();
   }
 
-  folly::Future<T> toFuture() && {
-    auto executor = promise_->executor_;
-    return SemiFuture<T>::fromAwaitable(std::move(*this)).via(executor);
-  }
+  auto toFuture() &&;
 
   ~Future() {
     if (!promise_) {
@@ -118,5 +115,23 @@ class Future {
 
   Promise<T>* promise_;
 };
+
+namespace detail {
+SemiFuture<Unit> toSemiFuture(Future<void> future) {
+  co_await future;
+  co_return folly::unit;
+}
+
+template <typename T>
+SemiFuture<T> toSemiFuture(Future<T> future) {
+  return SemiFuture<T>::fromAwaitable(std::move(future));
+}
+} // namespace detail
+
+template <typename T>
+auto Future<T>::toFuture() && {
+  auto executor = promise_->executor_;
+  return detail::toSemiFuture(std::move(*this)).via(executor);
+}
 } // namespace coro
 } // namespace folly

folly/experimental/coro/test/CoroTest.cpp

@@ -189,7 +189,7 @@ TEST(Coro, CurrentExecutor) {
   EXPECT_EQ(42, future.get());
 }
 
-coro::Task<int> taskTimedWait() {
+coro::Task<void> taskTimedWait() {
   auto fastFuture =
       futures::sleep(std::chrono::milliseconds{50}).then([] { return 42; });
   auto fastResult = co_await coro::timed_wait(
@@ -215,12 +215,12 @@ coro::Task<int> taskTimedWait() {
       std::move(slowFuture), std::chrono::milliseconds{100});
   EXPECT_FALSE(slowResult);
 
-  co_return 42;
+  co_return;
 }
 
 TEST(Coro, TimedWait) {
   ManualExecutor executor;
-  EXPECT_EQ(42, via(&executor, taskTimedWait()).toFuture().getVia(&executor));
+  via(&executor, taskTimedWait()).toFuture().getVia(&executor);
 }
 
 #endif

folly/futures/Future.h

@@ -840,6 +840,10 @@ class SemiFuture : private futures::detail::FutureBase<T> {
       return {};
     }
 
+    void return_value(const T& value) {
+      promise_.setValue(value);
+    }
+
     void return_value(T& value) {
       promise_.setValue(std::move(value));
     }